The present invention relates to a screw implant for securing a dental prosthesis, comprising a threaded part which can be screwed into a jaw, and an occlusal structural support having a conical head and an inside thread for an occlusal screw, by means of which a cap can be secured on the conical outer face of the head.
DE-OS 1,961,531 discloses such a screw implant whose structural support has a conical head on which a disk-shaped cap is screwed by means of a flatheaded screw. The flat surface of the screw head lies in the same plane as the likewise flat occlusal end face of the cap, in which connection, as a result of production tolerances, an annular gap between the screw head and the cap can scarcely be avoided in practice. The cap comprises a conical bore for receiving the screw head, and a corresponding conical surface is also provided for the screw head in the upper end of the threaded bore in the structural support. The outer face of the head of the structural support and, correspondingly, the inner face of the cap bearing thereon are conical, opening downwards to the thread of the structural support. The various conical surfaces, and also the end face of the structural support on which the cap bears with its edge, require an extremely high finishing accuracy. Problems arise above all with the oppositely disposed conical surfaces of the screw on the one hand, and of the outer face of the head of the occlusal structural support on the other hand. There is a considerable risk of gaps being formed between the head, cap and screw head. Such gaps represent a significant risk of development of infectious decay centers and are altogether disadvantageous with respect to the existing hygienic requirements.
DE-OS 2,413,883 discloses an endosteal implant for securing a firmly fitting dental prosthesis, which implant has a support comprising two sections. The one section is implanted into the jaw and comprises a blind bore extending substantially over the entire length and having an inside thread. The other section, which serves to secure a dental prosthesis, has a pin with an outside thread which can be screwed into the inside thread of the first section. Arranged between these sections is a flat, cylindrical ring of an elastic material, the thickness of this ring corresponding approximately to the thickness of the mucosa covering. Following the implantation of the lower section, instead of the second section, an auxiliary section is first screwed into the lower section, in which connection, by means of the elastic ring, it is intended to achieve a tight seal between the head of the auxiliary section and the lower section of the support. This auxiliary section is designed as a screw and remains in the lower section until the lower section has become completely incorporated into the jaw-bone. Thereafter the auxiliary section is removed and the second section of the support is screwed in its place into the lower section, so that, finally, the dental prosthesis can then be screwed to the second section.
Furthermore, a screw implant is described in the German periodical Die Quintessenz, die Monatszeitschrift fur den praktizierenden Zahnarzt (The Quintessence, the Monthly Magazine for the Practicing Dentist), November 1984, Report No. 6678, pages 1-11. This screw implant, which has proven satisfactory in practice for over 10 years, comprises a self-tapping threaded part which provides a large contact area over a short bone length and ensures optimum primary stability. By virtue of the titanium plasma coating of the threaded part, an enlarged contact area is obtained, this providing advantages with respect to the biocompatibility. The screw implant can be inserted transgingivally as a single implant, or several screw implants can be interlocked by means of a bar in order to construct a bar prosthesis. In this case, a cap is placed on the conical head and is secured by means of a screw engaged in the inside thread. The head has four continuous longitudinal grooves in its conical outer face which are used for the application of a tool when screwing into the jaw. Continuous grooves of this type have proven disadvantageous with respect to the hygienic requirements. The recessed-head screw to be screwed into the inside thread of the screw implant has a flat head which is countersunk below the top edge or end face of the cap. The end face of the cap is situated on an annular attachment inside which the screw head is arranged, which leads to deposits of filth and from which disadvantages arise with respect to hygienic suitability.